Many processing techniques for light waves require that the polarization state of the light wave be determined quickly and accurately. As used herein, the word "light" refers to an electromagnetic signal of any frequency. Because the polarization state of an incident light wave can vary with time, determination of the polarization state as time proceeds is a challenge, especially if the determination is to be made with only a small time delay.
In certain heterodyne and homodyne coherent optical communication schemes, the electrical field of the received light wave must be aligned with the optical field of a local oscillator to give maximum photodetector current. Fluctuation of polarization direction or state induced on the incident light wave during its propagation results in an unacceptable increase in the bit error rate for the message transmitted. In order to overcome the signal fading problems, certain polarization diversity apparatus has been proposed and demonstrated by various workers in the field. In these schemes, the light wave received is separated into two orthogonally polarized components and each component is heterodyned and detected separately. The two components are then amplified and summed to produce a polarization signal for further processing. In some of these apparatus, the relative phase shift between the two components is determined or corrected and the polarization angle is adjusted for maximum signal-to-noise ratio. Moreover, this is usually done over relatively long time intervals and requires relatively complex apparatus for this purpose.
In another field of application, electrical current measurements are made using fiberoptic sensors based on the Faraday rotation effect. When electric current flows in the vicinity of an optical fiber, the induced magnetic field in the fiber changes the state of polarization of a light beam propagating in the fiber. Analysis of this polarization rotation allows a determination of the magnitude of electrical current. These applications, and others as well, may use different types of polarization diversity receivers ("PDRs") that have been assembled using a variety of polarization beam splitter elements. These elements include polarizer beam splitter cubes, Glan-Thompson beam splitter polarizers and Wollaston prism polarizers. A beam splitter cube has the advantage of low cost and 90.degree. separation of the polarization components. However, the cube provides a relatively poor extinction ratio (about 100:1). The Wollaston prism and Glan-Thompson polarizing beam splitter have high extinction ratios (about 10.sup.5 :1) but have a small angular separation and are very costly due to incorporation of expensive optical elements therein.
In a PDR, the incident light wave is usually coupled through a lens or other optical focusing element to the polarizing beam splitter and is separated into two orthogonal polarization components, with each component falling on a separate optical detector. If the incident light wave is coupled into the PDR through a single mode fiber for application in high speed data communications and for fiber optic sensors, the alignment tolerances become very stringent, and optical components of relatively large size are no longer suitable.
Okoshi et al., in "Polarization-Diversity Receiver for the Heterodyne/Coherent Optical Fiber-Communications," The Fourth IOOC Paper, No. 30C3-2, June 27-30, 1983, Tokyo, Japan, discuss the use of a PDR together with two linearly polarized local oscillator beams to determine the relationship of two orthogonal polarization components of an incident light beam. This requires additional components such as half wave plates and requires that the ratio of power contained in two portions split from the light beam be controlled.
Jellison, in "Four-channel polarimeter for time-resolved ellipsometry," Optics Letters, vol. 12 (1987), pp. 766-768, describes recent measurements and apparatus made by many works in the field of polarization determination. These devices rely upon ellipsometry, two-channel polarimetry, four-channel polarimetry, phase retardation and other techniques to determine one or more of the parameters needed to specify the polarization state of a light beam. Kasovsky, in "Phase- and Polarization-Diversity Coherent Optical Techniques," Jour. of Lightwave Technology, vol. 7 (1989), pp. 279-322, reviews the advantages and disadvantages of several optical parameter diversity techniques.
Polarization beam splitters have been used for optical beam splitting and combining by previous workers. Use of a birefringent material as a polarization-dependent beam splitter is disclosed by Bergmann in U.S. Pat. No. 4,492,436. An optical beam with TE or TM polarization approaches the crystal at a large incidence angle. A beam with one of these two polarizations is reflected at the crystal, and a beam with the other of these two polarizations is transmitted by the crystal.
A polarization-dependent beam splitter, possibly two right angle prisms joined at the hypotenuse, disclosed in U.S. Pat. No. 4,653,867 issued to Urabe et al., is used to combine two optical beams with perpendicular polarizations with a resulting small angular offset.
Carlson et al., in U.S. Pat. No. 4,685,773, disclose a birefringent optical multiplexer/demultiplexer with flattened bandpass that uses a first polarization beam splitter to separate an optical beam into two component beams with perpendicular linear polarization vectors. The two component beams then propagate in parallel through a plurality of birefringent crystals, in order to alter the two polarization vectors relative to one another, and are recombined using a second polarization beam splitter.
U.S. Pat. No. 4,702,557, issued to Beckmann et al., discloses an optical branching device that uses a prism and a rhombohedral plate, both optically transparent and each having a planar face, with the two planar faces being parallel and facing one another. A thin film of birefringent liquid crystal is positioned between and contiguous to these two planar faces, with a refractive index n that matches the refractive index of only one of the prism material (n.sub.1) and the plate material (n.sub.2). The angle of incidence .THETA. of the optical beam on the liquid crystal film is selected so that sin .THETA.&gt;n.sub.i /n (n.sub.i &lt;n; i=1 or 2). One of two perpendicular linear polarization components of an optical is completely reflected, and the other polarization component is transmitted with little or no reflection losses. This device separates an optical beam into two component beams with perpendicular linear polarization vectors.
Mochizuki et al., in U.S. Pat. No. 4,720,162, disclose use of a polarization beam splitter to receive light beams from two light sources and to issue a single output light beam propagating in a predetermined direction. The two input light beams arrive at the beam splitter from perpendicular directions, only one of the two light sources is operative at a given time, and the two incoming light beams are assumed to each be linearly polarized with perpendicular polarization vectors.
An optical switching device employing a polarization-maintaining input optical coupler and a birefringent crystal is disclosed by Byron in U.S. Pat. No. 4,761,050. The birefringent crystal acts as a polarization beam splitter to provide two optical propagation paths therein, one path for each of two component beams having perpendicular linear polarization vectors. Only one of the two polarization component beams issues from the system, depending upon whether the optical coupler receives, or does not receive, an externally controlled, switchable pump signal.
What is needed here is apparatus for determination of the instantaneous polarization state of an incident light wave that is accurate and fast enough to follow the variation in polarization state of the light wave. Preferably, the operational part of the apparatus should be relatively small in size (linear dimension less than a few cm) and be contained in a single chip, and the apparatus should be relatively simple, with few or no moving parts that can become misaligned. The hybrid should be contained on a single substrate and should include, if desired, the electronics for signal processing.